The production of ethers by the reaction of an isoolefin and an alcohol is a well-known commercial operation. A number of detailed descriptions of such processes, particularly as they relate to the production of methyl tert.-butyl ether, are contained or referenced in recently issued U.S. Pat. No. 4,447,653 (B.V. Vora) and in the Hanes et al. U.S. Pat. No. 4,418,219 issued Nov. 29, 1983, both of which are incorporated by reference herein. Such procedures are, in general, also applicable to the production of methyl tert.-amyl ether.
Both methyl tert.-butyl either (MBTE) and methyl tert.-alkyl ether (MTAE) are useful as high octane blending agents for gasoline motor fuels by virtue of their high Research Octane Number (RON) of about 120. Perhaps the most commonly employed reaction in the preparation of MTBE and MTAE is that between methanol and isobutylene or isoamylene, respectively. A wide variety of catalyst materials have been found to promote this reaction including ion-exchange resins such as a divinylbenzene cross-linked polystyrene ion exchange resin in which the active sites are sulfuric acid groups: and inorganic heterogeneous catalysts such as boric acid, bismuth molybdate, and metal salts of phosphomolybdic acids wherein the metal is lead, antimony, tin, iron, cerium, nickel, cobalt or thorium. Also boron phosphate, blue tungsten oxide and crystalline aluminosilicates of the zeolitic molecular sieve type have also been proposed as heterogeneous catalysts for the reaction of methanol and isobutylene.
The preference for the isoalkylene-methanol reaction is in part, at least, due to the relative abundance of the starting materials. Both isobutylene and isoamylene are readily available in a petroleum refinery from both fluid catalytic crackers and as a by-product of ethylene production. Methanol is, of course, a staple commercial chemical of long standing. Moreover, isobutylene, because of its volatility, cannot be added to the gasoline pool without alkylation. Methanol cannot be added to gasoline in significant quantities because of immiscibility problems and because of its corrosiveness toward existing internal combustion engines. The combining of these two compounds thus appears to be an advantageous way to extend the gasoline pool. The modification of a gasoline by the conversion of 2-methyl-1-butene and 2-methyl-2-butene to methyl tert.-amyl ether is proposed in U.S. Pat. No. 3,482,952.
In addition to being useful in the preparation of high octane ethers for gasoline up-grading, the etherification process is also useful as a separation process. The reaction of methanol with mixed C.sub.4 and C.sub.5 olefins is selective for isobutylene and isoamylene. Therefore, a mixed butylene and/or amylene stream common to refineries can use the aforesaid etherification process to separate this mixture and to produce a stream of essentially pure normal butenes and/or amylenes and essentially pure MTBE and/or MTAE. The ethers can subsequently be cracked to produce essentially pure isoalkylenes.
A wide variety of reaction conditions have heretofore been proposed for carrying out the reaction of isobutylene or isoamylene with methanol, depending in part upon the type of catalyst employed in each case. Thus, both vapor phase and liquid phase processes are known in which reaction temperatures are from about 50.degree. C. to about 400.degree. C., pressures vary from atmosphere to 1,500 psig, and the mole ratios of methanol to isoalkylene range from 0.1:1.0 to about 10:1. Both batch type and continuous process schemes are said to be suitably employed.
It is commonly the case that the source of isobutylene is a mixed C.sub.4 hydrocarbon stream from a refinery operation, and the reaction with methanol is carried out in the liquid phase at a temperature not exceeding 100.degree. C. The quantity of the MTBE produced depends upon the isobutylene content of the C.sub.4 hydrocarbon stream used. When a C.sub.4 hydrocarbon stream cut from steam cracking is used, providing a feedstock with approximately 50% isobutylene after butadiene extraction, the reactor effluent can contain almost 60% MTBE and can sometimes be used as a gasoline component without further treating. It is generally more desirable, however, to separate the unreacted C.sub.4 s from the reactor effluent by distilling off the unconverted C.sub.4 s. When this is done, MTBE of about 98% purity can be produced at an isobutylene conversion of approximately 95%. A further increase in conversion can be achieved only by using a higher methanol/isobutylene ratio in the reactor feedstock. Because greater than stoichiometric amounts of methanol are used in the high conversion MTBE processes (also to allow for fluctuating isobutylene concentrations), additional steps have to be included in such processes to recover the excess methanol from reactor effluent. The recovered methanol is then recycled to the reactor feed stream.
When the source of isobutylene is a mixed C.sub.4 hydrocarbon stream from a fluidized catalytic cracking (FCC) process, the isobutylene usually constitutes only about 12 to 16% of the total C.sub.4 's. With such feedstocks, the economics of the etherification process favors the use of a substantial excess of methanol, thereby increasing the importance of effective recovery and recycle of unreacted methanol.
In the prior known high conversion MTBE processes, the excess methanol is usually recovered by a combination of distillation and water washing steps. The reactor effluent containing MTBE, unconverted methanol and C.sub.4 hydrocarbons is fed to a debutanizer tower wherein the unreacted C.sub.4 's are distilled off as the overhead stream together with some methanol. The methanol from this stream is recovered by a water wash step and subsequent distillation of the resulting water-methanol solution. The mixed C.sub.4 hydrocarbons are routed to an appropriate processing point in the refinery such as an alkylation unit. The recovered methanol is recycled back to reactor feed. The bottoms from the debutanizer tower contain MTBE and methanol. This stream can, if desired, be distilled in a tower to recover MTBE product as the bottoms. The methanol-MTBE azeotrope (approximately 10 wt. % MEOH) is recovered overhead and recycled back to reactor feed thereby completing methanol recovery.
It will be understood by those skilled in the art that the foregoing observations concerning the commercial sources of isobutylene and the general procedures for converting this C.sub.4 isoalkylene to MTBE apply with equal validity to isoamylene and its conversion to MTAE provided the corresponding C.sub.5 hydrocarbons are substituted for the C.sub.4 hydrocarbons. For this reason, it will also be understood that whereas the processes of the present invention are hereinafter illustrated with reference to C.sub.4 hydrocarbons and MTBE, the illustrations also serve to describe the processes in which C.sub.5 hydrocarbons are utilized in the ultimate production of MTAE. Also mixed C.sub.4 and C.sub.5 hydrocarbon streams, feedstock and the like can also be substituted for their pure C.sub.4 and C.sub.5 counterparts.